Dinosaurs' 'bulletproof' armour revealed

An in-depth study of dinosaur armour has revealed an unexpected new level of strength, with some plates having a weave of fibres resembling today's bulletproof fabrics. The likely strength of such plates makes the dinosaurs studied - ankylosaurs - perhaps the best-protected creatures to have ever stalked the Earth.

Ankylosaurs were massive herbivores that grew up to 10 metres in length during the late Jurassic and Cretaceous periods. The coin-sized plates sported by the ankylosaurs fully covered their back, neck, head and even protected their eyes.

"Their whole lifestyle was connected with this armoured living, which improved defence against predators. Other dinosaurs would have to be really hungry to attack an ankylosaur," says lead researcher Torsten Scheyer at the University of Bonn, Germany.

Like other armoured animals - such as crocodiles and turtles - the bonelike plates were actually derived from skin and called osteoderms. While scientists have been fascinated with dinosaur armour for years, few studies have looked at the microstructure of their constituent minerals and proteins.

Collagen fibres

Scheyer, along with his doctoral supervisor Martin Sander, took samples of body plates from each of the three groups of ankylosaur species: polocanthids, thought to be the earliest ankylosaurs, nodosaurids and ankylosaurids. For comparison, armour plates from a stegosaur species and an ancient crocodile were also examined.

The researchers viewed thin sections of each plate with polarised light to highlight the mineralised traces left by bone and by fibres of collagen. Collagen is the tough protein that anchors armour plates to skin and also provides a framework on which the plates can grow.

Compared to stegosaurus or crocodile plates, the polocanthids had extra collagen fibres that may have stabilised the edges of individual plates. But in nodosaurids - which also had plates between 2 and 5 centimetres thick, the pattern of collagen fibres was highly organised in three dimensions.

They had a sets of structural fibres running parallel and perpendicular to the surface, and then further sets at 45° to each of these axes, providing strength in all directions. The fibres of the bulletproof fabric Kevlar are similarly arranged.

This type of arrangement is very strong and helps prevent bone from cracking, says John Currey at York University, UK - an expert in bone structure and mechanics. But he notes that assessing the actual strength of fossilised bone solely from their design is very difficult.

Crushing forces

Ankylosaurids had thinner plates, just 0.5 to 1.0 cm thick, but the undersides were hollowed out, creating a dome shape which may have added strength. But, in contrast to the nodosaurids, the collagen fibres were randomly arranged.

Scheyer and Sander conclude that ankylosaur species evolved progressively better armour plates to withstand the great crushing forces applied by the hungry jaws of its predators.

But Kenneth Carpenter, an armoured dinosaur expert at the Denver Museum of Natural History, US, remains unconvinced by their conclusions. Not all ankylosaurid plates are thin and not all nodosaurid plates are thick, he says.

He adds that the orientation of collagen in a plate is determined largely by its position on the skin, and Scheyer and Sander used plates whose original location on the dinosaurs' bodies were not known.

However, John Rensberger, who studies the structure of dinosaur bones at the University of Washington in Seattle, US, says the evolution of increasingly strong armour fits with the story of ankylosaurs, who became increasingly massive and slow moving as their amour grew to completely cover their bodies.

"More primitive relatives of ankylosaurs have osteoderms, though not enough to cover their body entirely. But these were longer limbed, faster moving dinosaurs," he says. "There's probably a co-evolution of the ankylosaurs, acquiring better armour at the expense of their speed."

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